simplify Datafield::flatVector: copy vector at once

Device/Data/Datafield.cpp

@@ -154,10 +154,7 @@ bool Datafield::hasSameShape(const Datafield& other) const
 
 std::vector<double> Datafield::flatVector() const
 {
-    std::vector<double> result(size());
-    for (size_t i = 0; i < size(); ++i)
-        result[i] = (*this)[i];
-    return result;
+    return m_values;
 }
 
 void Datafield::scale(double factor)